The present invention relates to wireless transmitters and receivers for transmitting and receiving digital data and, in particular, to transceiver systems that encode and decode data in a manner that takes advantage of different error rates of different bit positions in transmitted symbols.
Sophisticated wireless transceivers, used in mobile devices such as cell phones and computers, transmit digital data encoded into a physical signal such as a radio wave. This encoding processes multiple bits as a data unit into “symbols” which describe discrete states of the physical parameters of the transmitted signal. For example, in Quadrature Amplitude Modulation, (QAM) discrete amplitudes of two orthogonal sinusoidal waves are used in combination to create many different symbols of a “symbol constellation”. QAM can provide for different constellation sizes, e.g., 16 symbols in 16-QAM (distinguishing four amplitude levels), 64 symbols in 64-QAM (distinguishing eight amplitude levels), and 256 symbols in 256-QAM (distinguishing 16 amplitude levels).
At the receiver, the symbols are decoded into the multi-bit data units by matching the physical parameters of the received signals to the discrete states of symbols in the constellation. In QAM, the amplitudes of the orthogonal sine waves are measured and the symbol having the closest amplitude is assumed to have been transmitted.
When data is being transmitted in a noisy environment, it may be impossible to distinguish among the necessary amplitude levels of many symbols and a large constellation, for example, among the 256 symbols of 256-QAM. In these cases, it is known to adjust the transmitter and receiver to operate with a smaller constellation, for example 64-QAM, to provide greater distance between the symbols and thus more robust decoding in the face of noise. Alternatively or in addition, it is known to reduce the effective data rate of the transmission in order provide redundant data transmission, for example, using diversity to provide for redundant transmission channels or longer error detection and correction codes providing for effectively greater data redundancy.
If the received radio signal cannot be correctly decoded into a symbol, the symbol is discarded by the decoding process. Typically, incorrect reception is signaled by error detection codes transmitted with the data, which indicate a corruption of that data.